It is known that pyrite (FeS.sub.2) as usually found in coal has a relatively low magnetic susceptibility. However, when coal containing pyrite is hydrogenated in a reactor under elevated temperature and pressure conditions, it changes form and the magnetic susceptibility of the resulting iron sulfides initially increases over that of the form originally present in the coal. Upon further hydrogenation the iron sulfides continue to change form and their magnetic susceptibility reaches a peak value and then declines.
As described in U.S. Pat. No. 3,519,555 to Keith et al, the hydrogenation of coal to more valuable products (both liquids and gas) with achievement of better than 80 percent conversion of the coal, producing in the order of four barrels of oil per ton of coal, makes available an abundant alternative fuel source for commercially competitive fuels. It is recognized, however, that such liquid (coal-derived oil) usually contains considerable sulfur associated with unconverted coal solids and ash which together frequently amount to 15 to 20 wt. % of the coal. The sulfur-containing solids material portion must be substantially removed from the oil if it is to have commercial value and meet anti-pollution requirements.
The removal of metallic contaminants from coal and ores by magnetic separation means is well known. For example, it is well known in the water washing of coal to utilize magnetic separation for removing metal such as tramp iron. In the Leeman U.S. Pat. U.S. No. 2,998,882, it is suggested that magnetic separations of both coarse and fine particles can be accomplished to remove magnetite. In U.S. Pat. No. 3,463,310 to Ergun, it is suggested that electromagnetic heating of the iron particles in coal tends to convert them to a magnetic form. Ergun proposes to use microwave energy to convert pyrites to pyrrhotite, magnetite, etc. before magnetic separation.
The removal of ash solids from hydrogenated coal liquids by magnetic separation means is taught by U.S. Pat. No. 3,725,241 to Chervenak. However, it is desirable to enhance the magnetic susceptibility of sulfur-containing solids so that their removal can be achieved more easily and effectively at lower magnetic field strengths, thus producing low-sulfur liquid and char products.